Movements forming timepiece assemblies as defined in the field of the invention have been proposed in some prior documents. The patent CH 597 636, published in 1977, proposes such a movement with reference to FIG. 3 thereof. The movement is equipped with a balance-hairspring and a conventional maintenance device comprising a pallet assembly and an escape wheel kinematically linked with a barrel equipped with a spring. This timepiece movement comprises a device for regulating the frequency of the mechanical oscillator. This regulating device comprises an electronic circuit and a magnetic assembly formed from a flat coil, arranged on a support arranged under the felloe of the balance, and from two magnets mounted on the balance and arranged close to one another so as to both pass over the coil when the oscillator is activated.
The electronic circuit comprises a time base comprising a quartz generator and serving to generate a reference frequency signal FR, this reference frequency being compared with the frequency FG of the mechanical oscillator. The frequency FG of the oscillator is detected via the electrical signals generated in the coil by the pair of magnets. The regulating circuit is suitable for momentarily inducing a braking torque via a magnetic magnet-coil coupling and a switchable load connected to the coil. The document CH 597 636 provides the following teaching: “The resonator formed should have a variable oscillation frequency according to the amplitude on either side of the frequency FR (isochronism error)”. It is therefore taught that a variation in the oscillation frequency of a non-isochronous resonator is obtained by varying the oscillation amplitude thereof. An analogy is made between the oscillation amplitude of a resonator and the angular velocity of a generator comprising a rotor equipped with magnets and arranged in a gear train of the timepiece movement in order to regulate the operation thereof. As a braking torque reduces the rotational speed of such a generator and as such the rotational frequency thereof, it is herein merely envisaged to be able to reduce the oscillation frequency of an obligatorily non-isochronous resonator by applying a braking torque reducing the oscillation amplitude thereof.
To perform electronic regulation of the frequency of the generator, or of the mechanical oscillator, it is envisaged in a given embodiment that the load is formed by a switchable rectifier via a transistor which loads a storage capacity during braking pulses, to retrieve the electricity so as to supply the electronic circuit. The consistent teaching given in the document CH 597 636 is as follows: When FG>FR the transistor is conductive; a power Pa is then drawn from the generator/oscillator. When FG<FR, the transistor is non-conductive; therefore, power is no longer drawn from the generator/oscillator. In other words, regulation is merely performed when the frequency of the generator/of the oscillator is greater than the reference frequency FR. This regulation consists of braking the generator/oscillator with the aim of reducing the frequency FG thereof. As such, in the case of the mechanical oscillator, those skilled in the art understand that regulation is only possible when the barrel spring is strongly armed and that the free oscillation frequency (natural frequency) is greater than the reference frequency FR, resulting from a voluntary isochronism error of the selected mechanical oscillator. Therefore, there is a two-fold problem, i.e. the mechanical oscillator is selected for that which is usually an error in a mechanical movement and the electronic regulation is only functional when the natural frequency of this oscillator is greater than a nominal frequency.
In conclusion, the teaching generally given to those skilled in the art is as follows: If it is sought to electronically regulate the frequency of a balance-hairspring of a conventional timepiece movement, it is necessary to change the balance-hairspring in order to firstly arrange at least one magnet on top and secondly to modify the natural frequency thereof such that this natural frequency is greater than the frequency sought. The consequence of such a teaching is obvious: It is necessary to misadjust the mechanical resonator so that it oscillates at an excessively high frequency so as to enable the regulation device to consistently return the frequency thereof to a lower frequency, corresponding to the theoretical frequency sought, by a succession of braking pulses. Consequently, the resulting timepiece movement is voluntarily set in such a way that precise operation is dependent on the electronic regulation, failing which such a timepiece movement would have a very significant time drift. As such, if for one reason or another the regulation device is deactivated, particularly due to damage, then the watch equipped with such a movement will no longer be precise, to an extent that it is actually no longer operational. Such a situation is problematic.
The use of a magnet-coil type electromagnetic system for coupling the balance-hairspring with the electronic regulation circuit gives rise to various problems. Firstly, the arrangement of permanent magnets on the balance results in a magnetic flux being consistently present in the timepiece movement and said magnetic flux varying spatially periodically. Such a magnetic flux may have a harmful effect on various members or elements of the timepiece movement, particularly on elements made of magnetic material such as parts made of ferromagnetic material. This may have repercussions on the proper operation of the timepiece movement and also increase wear of pivoted elements. It may indeed be envisaged to shield to a certain extent the magnetic system in question, but shielding requires specific elements which are borne by the balance. Such shielding tends to increase the size of the mechanical resonator and the weight thereof. Furthermore, it limits the possibilities of clean visually appealing configurations. Furthermore, a high-intensity external magnetic field may damage the magnetised elements of the electromagnetic system.
Those skilled in the art are aware of the proposed embodiments of mechanical timepiece movements, comprising a device for regulating the frequency of the balance-hairspring, where it is envisaged to act upon the oscillating balance with an electromechanical system formed, on one hand, by a stop which is arranged on the balance and, on the other, by an actuator equipped with a movable finger which is actuated at a defined braking frequency in the direction of the abutment. This concept is intended to synchronise the frequency of the oscillator against that of a quartz oscillator with a claimed interaction between the finger and the stop when the mechanical oscillator exhibits a time drift relative to the quartz oscillator, the finger either momentarily locking the balance which is then stopped in the movement thereof during a certain time interval (the stop bearing against the finger moved in the direction thereof upon the return of the balance towards the neutral position thereof), or limiting the oscillation amplitude when the finger arrives against the stop while the balance rotates in the direction of the maximum amplitude position thereof.
Such a regulation system has numerous drawbacks and it could seriously be doubted that it could form an operational system. The ‘blind’ action of the finger relative to the movement of the stop and to any potential initial phase shift of the oscillation of the stop in relation to that of the finger poses multiple problems. The action is limited to an angular position given by the position of the actuator relative to the balance-hairspring. As such, the effect of the interaction between the finger and the stop is dependent on the oscillation amplitude of the balance-hairspring and on the position of the actuator. In conclusion, such embodiments appear to a person skilled in the art to be highly unlikely, and such a person skilled in the art is deterred from such a use. Moreover, those skilled in the art of the present invention are not aware of watches equipped with such an electromechanical system that have been introduced onto the market.